Inhibitors of aminopeptidase enzymes have been prepared by the synthesis of peptide substrate analogues in which the scissile amide bond has been replaced with the hydrolytically stable ketomethylene (-COCH2-) and hydroxyethylene [-CH(OH)CH2-] functionalities. Two synthetic strategies were used to prepare the inhibitors, and the advantages and disadvantages of each are discussed. The synthesis of peptides that contain the hydroxyethylene isostere was complicated by competing lactone and lactam formation, and attempts to prepare free N-terminal dipeptide hydroxyethylene isostere derivatives were unsuccessful. All ketomethylene isosteres examined were weak inhibitors of both leucine aminopeptidase and aminopeptidase M. However, the ketomethylene inhibitor LysK(RS)Phe (58) (Ki = 4 nM) is a potent inhibitor comparable to the natural product, arphamenine A (ArgKPhe; Ki = 2.5 nM). Normal Michaelis-Menten kinetics for inhibition of membrane leucine aminopeptidase are observed in the absence of magnesium ion, but nonlinear kinetics were obtained in the presence of Mg2+.